1. Field of the Invention
The present invention relates to a fabrication method of a solid electrolytic capacitor and more particularly, to a fabrication method of a solid electrolytic capacitor using a conductive polymer (for example, polypyrrole, polythiophene, and polyaniline) as a solid electrolyte.
2. Description of the Prior Art
A solid electrolytic capacitor of a chip type has a structure shown in FIG. 1.
In FIG. 1, a porous capacitor body or pellet 2, which is typically made by sintering a powder of a valve metal such as tantalum (Ta) and aluminum (Al), serves as an anode. An oxide layer (not shown), which is formed on the expanded surface of the porous capacitor body 2, serves as a dielectric. A solid electrolyte (not shown), which is formed on the oxide layer, serves as a cathode.
An anode wire 1 is implanted into the top face of the body 2. One end of an anode lead 10 is connected to the anode wire 1.
A conductive layer (not shown) is formed on the solid electrolyte to cover the entire surface of the solid electrolyte. One end of a cathode lead 11 is fixed onto the conductive layer by using a conductive adhesive 9.
The capacitor body 2, the oxide layer, the solid electrolyte, the conductive layer, and the anode wire 1 constitute a capacitor element.
The capacitor element is encapsulated by an epoxy resin package 12 in such a way that the capacitor element and the ends of the anode and cathode leads 10 and 11 are buried in the package 12. The parts of the anode and cathode leads 10 and 11 protruding from the package 12 are bent along the surface of the package 12.
The solid electrolyte has a function of electrically interconnecting the cathode lead with the entire surface of the dielectric formed on the capacitor body 2. Therefore, from this viewpoint, it is desirable that the solid electrolyte is a substance having a high electrical conductivity. On the other hand, the solid electrolyte needs to have a healing function for healing an electrical short due to a defect in the dielectric.
Accordingly, a metal, which has a high electrical conductivity, but has no dielectric healing function, cannot be used as the solid electrolyte. As a result, conventionally, a compound such as manganese dioxide (MnO.sub.2) has been used as the solid electrolyte, because MnO.sub.2 has a property that it is transformed from a conductor into an insulator due to the heat generated by a short-circuit current.
Further, the solid electrolyte is usually subjected to heat at a temperature as high as 240 to 260.degree. C. during a mounting process of the solid electrolytic capacitor on a mounting board (for example, a printed wiring board). Accordingly, the solid electrolyte needs to have a heat resistance against the heat at a temperature of 260.degree. C. or higher.
The previously-described MnO.sub.2 has such a high heat resistance property as above and therefore, it is well suited for use as the solid electrolyte for the solid electrolytic capacitor.
Thus, any material to be used as the solid electrolyte for a solid electrolytic capacitor needs to meet the following three requirements: (i) high electrical conductivity property; (ii) dielectric healing function; and (iii) heat resistance property of 260.degree. C. or higher.
The manganese dioxide that has been favorably used as the solid electrolyte is provided with sufficient properties in the points of (ii) the dielectric healing function and (iii) heat resistance. However, the manganese dioxide has a relatively low electrical conductivity of approximately 0.1 S/cm. Thus, the manganese dioxide cannot be said to be sufficient in the point (i).
Then, in recent years, various capacitors using one of such conductive polymers as polypyrrole, polythiophene, and polyaniline as the solid electrolyte have been vigorously developed. This is because these conductive polymers further have an electrical conductivity as high as 10 to 100 S/cm, which meet the above requirements (i), (ii), and (iii).
Generally, with an electrolytic capacitor utilizing a conductive polymer of this sort, there are three requirements to be met concerning its formation.
A first requirement is that the conductive polymer needs to be formed on the surface of the oxide layer within the pores of the capacitor body 2 with no omission.
A second requirement is that the conductive polymer needs to have a specific thickness or greater on the external surface of the porous body 2.
A third requirement is that good electrical and mechanical connection needs to occur between the layer of the conductive polymer and the conductive layer formed thereon. The conductive layer has, for example, a two-layer structure consisting of a graphite sublayer and a silver paste sublayer located on the graphite sublayer.
To meet the above first and second requirements, the following improved method was developed, which is disclosed in the Japanese Non-Examined Patent Publication No. 63-173313 published in 1988. This improved method includes a first step of forming a first conductive polymer layer as a precoat layer due to chemical polymerization, and a second step of forming a second conductive polymer layer on the precoat layer due to electrolytic polymerization, resulting in a solid electrolyte with a two-layer structure.
The method disclosed in the Japanese Non-Examined Patent Publication No. 63-173313 offers an advantage that the solid electrolyte can be conveniently formed by electrolytic polymerization. However, since the surface of the second conductive polymer layer formed by electrolytic polymerization has a low degree of irregularities, the above third requirement of good electrical and mechanical connection cannot be met with ease.
Another improved method is disclosed in the Japanese Non-Examined Patent Publication No. 4-369819 published in 1992. This method is able to control the thickness of a conductive polymer layer as the solid electrolyte to meet the above second requirement.
With the improved method disclosed in the Japanese Non-Examined Patent Publication No. 4-369819, an oxidizing agent is sprayed toward a porous capacitor body in the formation process of a conductive polymer layer. This method has not only excellent controllability for the thickness of the conductive polymer layer but also an effect of decreasing the amount of the necessary oxidizing agent.
However, with the method of the Japanese Non-Examined Patent Publication No. 4-369819, since no irregularities are formed on the surface of the conductive polymer layer, the third requirement of electrical and mechanical connection is not met.
Thus, either of the above conventional methods disclosed in the Japanese Non-Examined Patent Publication Nos. 63-173313 and 4-369819 leaves room for improvement in the third requirement about the electrical/mechanical connection of the conductive polymer layer with a conductive layer to be formed thereon in a subsequent process.
Then, to solve the above third requirement, a technique of forming a conductive polymer layer was developed, which is disclosed in the Japanese Non-Examined Patent Publication No. 7-94368 published in 1995. With the technique, irregularities are formed on the surface of a conductive polymer layer to thereby increase its surface area, improving the adhesion strength of the conductive polymer layer with a conductive layer to be formed thereon.
FIGS. 2 and 3 are enlarged views of the part A in FIG. 1, respectively, in which, the reference numeral 3 denotes the oxide layer formed to cover the entire surface of the capacitor body 2.
With the method of the Japanese Non-Examined Patent Publication No. 7-94368, a first conductive polymer layer 4 is formed on the oxide layer 3 to bury the fine pores of the body 2, and then, a fine conductive or insulating powder 5 is attached onto the first conductive polymer layer 4. Subsequently, a second conductive polymer layer 6 is formed on the first conductive polymer layer 4 to cover the powder 5, providing irregularities on the surface of the second conductive polymer layer 6.
Alternately, as illustrated in FIG. 3, a first conductive polymer layer 4 is formed on the oxide layer 3 to bury the fine pores of the body 2, and then, a second conductive polymer layer 6 is formed on the first conductive polymer layer 4 in such a way that a fine powder 5 is attached onto the first conductive polymer layer 4.
Following the process of forming the second conductive polymer layer 6, a graphite sublayer 7 and a silver paste sublayer 8, which constitute the conductive layer formed on the solid electrolyte, are successively formed on the second conductive polymer layer 6.
The surface irregularities of the second conductive polymer layer 6 are realized by using a solution in which the fine powder 5 is mixed and dispersed or suspended. Specifically, after forming the first conductive polymer layer 4 on the oxide layer 3, the capacitor body 2 is immersed into a flowing solution in a container where the fine powder 5 is suspended or dispersed in the solution. Thus, the fine powder 5 is deposited on the first polymer layer 4.
Alternately, the fine powder 5 is mixed with a solution of a monomer or a solution of an oxidizing agent in advance. Then, after forming the first conductive polymer layer 4 on the oxide layer 3, the second conductive polymer layer 6 is formed by chemical oxidative polymerization using the solutions of the monomer and the oxidizing agent. During this polymerization process, the fine powder 5 is deposited on the first polymer layer 4.
Further, when the second conductive polymer layer 6 is formed by electrolytic oxidative polymerization, a solution of an electrolyte, in which the fine powder 5 with an electrical conductivity is mixed therewith, is used. The powder 5 is absorbed into the second conductive polymer layer 6 during the electrolytic oxidative polymerization.
By using any one of the three irregularity-formation methods, the irregularities are formed on the surface of the second conductive polymer layer 6, thereby realizing the layer 6 with an adhesion property high enough to meet the above third requirement.
In the above three processes of forming the surface irregularities of the second conductive polymer layer 6 in the method of the Japanese Non-Examined Patent Publication No. 7-94368, it can be said that the solution containing the dispersed fine powder 5 is used while staying in the state of liquid.
With the conventional method disclosed in the Japanese Non-Examined Patent Publication No. 7-94368, the adhesion property (i.e., the electrical and mechanical connection) between the second conductive polymer layer and the graphite sublayer 7 formed thereon can be improved. However, there arises the following problems.
A first problem is that the amount of the fine powder 5 deposited on the first conductive polymer layer 4 tends to greatly change depending upon the speed at which the capacitor body 2 is pulled up from the solution containing the fine powder 5.
A second problem is that the amount of the fine powder deposited on the first conductive polymer layer 4 may greatly fluctuate according to the deposited location of the capacitor body 2. This problem is caused by the fact that the state of dispersion of the fine powder 5 in the solution is unstable.
A third problem is that the amount of the fine powder deposited on the first conductive polymer layer 4 is difficult to be kept unchanged for a long time. This problem is caused by the fact that if the solution containing the fine powder 5 is continuously used, the content of the powder 5 in the solution is decreased. In other words, the degree of the irregularities of the second conductive polymer layer 6 tends to fluctuate with the locations of the body 2, with the bodies 2 processed in the same lot, and with the lots.
The variation or fluctuation of the amount of the deposited fine powder 5 will cause the following disadvantages.
Specifically, if the amount of the deposited powder 5 is too small, the desired degree of the irregularities is not formed on the surface of the second conductive polymer layer 6. Contrarily, if the amount of the deposited powder 5 is too large, the irregularities once formed are canceled finally. As a result, the adhesion strength between the second conductive polymer layer 6 and the graphite sublayer 7 of the conductive layer formed thereon decreases and accordingly, the Equivalent Series Resistance (ESR) and tan .delta. cannot be satisfactorily decreased by the formation of the irregularities.